Environment: Study says the lake's noted clarity could be irreversibly doomed without a major effort to curb elements linked to pollution, construction.

SACRAMENTO — Lake Tahoe's legendary clarity could be irreversibly doomed within a decade without a heroic effort to stem the growth of algae that is turning its waters a murky green, according to a scientific study to be released today.

"Time is short," concludes the executive summary of the 1,200-page report. "Lake Tahoe is gravely imperiled."

Conducted by government scientists and university researchers in California and Nevada, the long-awaited study comes three years after President Clinton held a lakeside summit focusing national attention on the Tahoe Basin.

The report says Tahoe's clarity continues to decline at a rate of about 1 foot per year. In the 1960s, a white disc resembling a dinner plate used to measure Tahoe's clarity was visible at depths of 105 feet. Today, that same plate is visible only as deep as 66 feet, and might be seen only as deep as 40 feet by 2030 if current trends continue, the report said.

"The situation is urgent," said Thomas Cahill, an atmospheric scientist at UC Davis and member of the research team. "And it's going to take some dramatic measures to turn things around."

The algae's clouding of the lake is caused by nitrogen, phosphorus and other nutrients that enter through a variety of routes. Cahill said more than half of the nutrients come from the air. Among the sources are wood smoke, diesel exhaust and sand that is applied to roads in winter, then gets ground into a fine powder that winds up in the lake when snow melts.

Construction is also to blame. In 1960, just 500 houses ringed the lake. Twenty years later, there were 19,000 homes. Runoff from development enters the lake through ground water and tributaries that feed the lake.

Once the nutrients are in the water, they remain there for decades, overwhelming the lake's ability to cleanse itself, said John Reuter, a lake ecologist who has studied Tahoe for 22 years.

"The lake is so deep and so big, a lot of these pollutants that affect the clarity can actually stay in there and be recycled for 10 or 20 years," Reuter said. "So it's almost like compound interest in a bank account, where this stuff just continues to build up and build up faster than it can leave the lake."

Scientists say that unless urgent measures are taken now, the lake will become stratified within 30 years, with a warm algae-rich layer on top and a cold, sterile layer below it. Once stratification happens, it's virtually impossible to reverse.

Cahill said that the only way to stem the degradation is to reduce "the pollutions per person per day" afflicting the popular region. One way to achieve that, he suggests, is to curb vehicle use in the basin--much as Yosemite National Park has done--and transport people around by light rail or buses fueled by natural gas.

"We should also relocate major roadways, like Highway 50, away from the lake shore," Cahill said. "We need to create ecological buffers [between the roads and the lake], marshes that can capture runoff before it hits the lake."

One estimate by a regional planning group said basic erosion control measures and water treatment projects would cost more than $900 million and take a decade to construct.

Aside from the warning about clarity, the report addressed air-quality issues, finding that South Lake Tahoe is the only California city where ozone levels have risen during the past 20 years. In most cities, including Los Angeles, ozone has been on the decline.

Scientists concluded that the culprit behind Tahoe's problem is the rapid growth of suburbs east of Sacramento, particularly along the Interstate 80 and U.S. 50 corridors. Pollution generated there gets blown uphill on the afternoon breeze, and it settles over the lake.

Researchers also found that several animal species once abundant in the Tahoe basin have vanished or been lost. They include the Lahontan cutthroat trout, Sierra Nevada red fox, willow flycatcher and yellow-legged frog made famous by Mark Twain's story "The Jumping Frogs of Calaveras County."

The report was a $2.6-million collaborative effort by government scientists and scholars at UC Davis and the University of Nevada at Reno. The study breaks ground by analyzing data in a comprehensive way and creating an emergency road map for land-use decisions and environmental management.